1. Field of the Invention
This invention relates to a method and apparatus that facilitates the rapid, uniform temperature cycling of samples. More particularly, the invention is directed to an apparatus for performing DNA amplification.
2. Background
There are a variety of investigative settings in which many oligonucleotide or polynucleotide samples, or specific DNA fragments within a sample mixture, are amplified by polymerase chain reaction (PCR). For example, DNA samples contained in the wells of a microtiter plate can be PCR-amplified as an array. In still another setting, it may be desirable to compare the amplification products of one or more DNA fragments contained in different tubes in a tube holder.
If the amplified fragments from the different samples are to be compared, either for fragment size or quantity, it is desirable to conduct the PCR amplification of each sample under substantially identical conditions. This means that the concentration of PCR reagents, as well as the thermal cycling times and temperatures, should be carefully controlled and uniform among all of the samples.
Heretofore, a variety of devices have been used or proposed for carrying out PCR reactions simultaneously in a plurality of structures. Typically, these devices involve a heat block placed against the wells of a microtiter plate, or a heat block designed to hold a plurality of sample tubes. The block, in turn, is alternately heated and cooled by circulating a heating fluid through the block, or by heat conduction to the block. It is difficult to achieve uniform heating and cooling cycles in this type of device, due to uneven heat transfer rate and temperatures within the block and due to the difficulty of providing a good thermal connection between the block and the wells or tubes.
It has also been proposed to circulate a temperature-controlled fluid (such as air or water) past sample tubes as shown by U.S. Pat. No. 5,187,084 to Hallsby. This allows a higher frequency for temperature cycling as the temperature of the flowing fluid is easier to control than that of the block. However, this approach results in temperature gradients on the sample tubes because the fluid flow around a tube causes the temperature of the fluid flowing next to the sample tube to be affected by the temperature of the sample tube itself. Thus, the fluid flow adjacent to the tube at the upstream part of the tube is at a different temperature than the fluid adjacent to the tube at the downstream portion of the tube. In addition, temperature gradients occur within the sample tube because the heat transfer where the fluid impinges the tubes is different from the heat transfer where the fluid flows past the tubes.